Dielectric microspheres have recently drawn increasing attention as fluorosensors in sensing applications. In those sensors, the sensor surface is immobilized with a layer of molecules, such as antibodies, for the subsequent capture of analytes, such as antigens. In a direct assay configuration, antigens are conjugated with fluorescent dye molecules: when the antigen binds with the antibody on the sensor surface, the fluorescent molecule is held sufficiently close to the microsphere surface that it is excited by evanescent light circulating in the microsphere. In a sandwich-type configuration, the antigen is first bound to the antibody on the sensor surface, and then a second layer of antibodies, labeled with a fluorescent dye, is added to bind to the captured antigens. The fluorescent molecules bound to the second layer of antibodies are excited by the evanescent field arising from light propagating in the whispering gallery modes (WGMs) of the microsphere. The resulting fluorescence from the excited dyes is collected and used as an indicator of the antigen binding events.
The WGMs of the microsphere are associated with a high Q-factor, and so the intensity of light, when coupled into the WGMs, is enhanced, as compared to the input light. The degree of enhancement is proportional to the Q-factor. A narrow bandwidth, tunable semiconductor diode laser, having a sub-megahertz spectral linewidth, is typically used as the light source for exciting a WGM in a microsphere cavity. The bandwidth of the laser light is comparable to the bandwidth of a single WGM resonance. Therefore, when the laser is tuned to a particular WGM resonance, most of the coupled light falls within the resonant bandwidth, and so there is efficient coupling into the WGM resonance. The high cost of such a laser, however, has proved to be a significant obstacle to the widespread introduction of microsphere-based sensors in many applications.